Process for recovering phosphorus from sludge



April 1969 .J. A. HINKEBEIN 3,436,184

PROCESS FOR RECOVERING IHOSPHORUS FROM SLUDGE Filed Aug. 25. 1964 Sheetof z CONDENSING MEDLIUM H PHOS PHORUS CONTAiN! NG GASES //;/1,H\\\

UNCONDENSED GASES l OXIDIZING AGENT 5 Z- TEQNDESING CONDENSING I MEDIUMMEDIUM OUT 1 TREATED SLUDGE PHOSPHORUS OUT PHOSPHORUS OUT FIG. I

INVENTORI JOHN A. HINKEBEIN BY vzz/so. @Z/ a ATTORNEY April 1969 J. A.HINKEBEIN 3,436,184

PROCESS FOR RECOVERING PHOSPHORUS FROM SLUDGE Filed Aug. 25, 1964 Sheet3 of 3 CONDENSING MEDIUM CONTAINING OXIDIZING AGENT PHOSPHORUS 2CONTAINING GASES UNCONDENSED GASES cowoawsmc 3 MEDIUM OUT '7'" .7TREATED sLuocE OUT PHOSPHORUS OUT F IG. 3

INVENTORI JOHNA. HINKEBEIN ATTORNEY United States atent O US. Cl. 23-2237 Claims ABSTRACT OF THE DISCLOSURE The phosphorus content of sludge isreduced by ad mixing a water-soluble inorganic hexavalent oxy-chromiumcompound with the sludge to improve the separation of phosphoruscontained in the sludge.

This invention relates to processes for the production and recovery ofphosphorus and is directly concerned with decreasing the phosphoruscontent of sludge obtained as a by-product in phosphorus production and,more particularly, relates to processes for treating sludge to recoverthe phosphorus therefrom.

In the production of elemental phosphorus by the thermal reduction ofphosphatic materials with a reducing agent, such as coke or othercarbonaceous reducing agents, in the presence of a flux such as silica,the phosphorus-containing gases given off in the furnaces contain solidimpurities such as particles of slag and unfused constituents of thecharging materials which cause the formation of sludge when thephosphorus vapor is condensed to the liquid form. This sludge containsvarying but appreciable amounts of phosphorus which oftentimes rendersproblems in the disposal thereof as well as lowering the over-all yieldof phosphorus produced.

The sludge, as the term is used in the art and herein, is a somewhatpoorly defined emulsion or dispersion containing solid impurities, Waterand phosphorus in widely varying proportions with usually a densitybet-ween the density of phosphorus and the density of water. The type ofemulsion can vary from being primarily a phosphorus in water typeemulsion, that is, the phosphorus being the discontinuous phase and thewater being the continuous phase, to a water in phosphorus typeemulsion, that is, the water being the discontinuous phase and thephosphorus being the continuous phase. In general, the discontinuousphase is usually in the form of globular particles having fineparticulate impurities associated therewith, with the size of theparticles being generally in the range of from millimeter dimensions tomicron dimensions.

Heretofore, methods for decreasing the phosphorus content of the sludgehave been physical methods for separating the phosphorus from the sludgewith such methods as filtration, distillation, centrifuging, briquettingand the like being employed. However, such methods leave much to bedesired because of, among other things, such limitations as the expenseof the methods, the type of equipment necessary and the maintenancerequired thereon, and the quality of the phosphorus recovered. As can beappreciated, therefore, a method which uses a chemical treatment fordecreasing the phosphorus content of the sludge and thereby minimizes orobviates the limitations of the foregoing methods would represent anadvancement in this art.

It is, therefore, an object of this invention to provide an improvedprocess for the production and recovery of phosphorus.

It is another object of this invention to provide an improved processfor decreasing the phosphorus content of sludge obtained as alay-product in phosphorus production.

It is another object of this invention to provide an improved processwhich uses a chemical treatment for decreasing the phosphorus content ofsludge obtained as a by-product in phosphorus production.

It is a still further object of this invention to provide an improvedprocess which uses a chemical treatment for reducing the phosphoruscontent of the sludge and thereby minimizes or obviates the limitationsof the physical methods used heretofore.

It is a still further object to provide an improved process for therecovery of phosphorus from sludge obtained as a by-product inphosphorus production.

These, as well as other objects of this invention, are accomplished by aprocess which comprises treating the sludge with an oxidizing agent inorder to decrease the phosphorus content of the sludge, all of whichwill be more fully described hereinafter. The treatment can be appliedeither during the formation of the sludge or after formation and forpurposes herein the term treating the sludge is intended to cover thetreatment thereof either during or after its formation. As can beappreciated, such a process is extremely versatile in that it can beused at various stages within the process for the production ofphosphorus. In addition, the process is simple, economical and permitsthe recovery of a relatively high quality phosphorus. The use of anoxidizing agent in this manner is believed to be unexpected due to theease with which phosphorus can be oxidized. In theory at least, thediscontinuous phase is coated with such materials as oxide films,particulate impurities and the like which prevents the union orcoalescence thereof and the effectiveness of the oxidizing agent isbelieved to be attributable to its action in attacking the coating ofthe discontinuous phase and/ or inhibiting the formation of the coatingon the discontinuous phase with the result being an improved separationof the phosphorus and the sludge and the reduction of the phosphoruscontent of the sludge.

In general, any oxidizing agent can be used and, in particular,oxidizing agents whch function in acidic solutions, especially strongoxidizing agents including organic as Well as inorganic oxidizingagents. As examples, the followng are included: hydrogen peroxide andits addition compounds, including the hydroperoxides and peroxidecompounds, such as the peroxide of sodium and the superoxide ofpotassium, urea percompounds, perborates, and the peracids such aspersulfuric acid, per acetic acid, peroxy monophosphoric acid and theirWater-soluble salt compounds such as sodium, potassium, ammonium andorganic amine salts. Also included are such inorganic oxidizing agentsas the halogens (fluorine, chlorine, bromine, and iodine) and especiallychlorine as C1 or nascent chlorine C1, the acid and water-soluble saltscontaining oxygenated anions, such as N0 1 S604", MnO and especially theovygenated halide anions, such as, C10 1 10 C10 BrO ClO, and the like,and particularly the alkali metal salts, such as sodium, potassium,lithium and the like, and the ammonium salts thereof. The salts of theoxygenated anions can be activated, if desired, by use of a stronginorganic acid, such as a hydrohalide" acid (hydrochloric acid) and thelike. Water-soluble hexapositive chromium compounds are preferredbecause of the better yields and quality of phosphorus recovered fromthe sludge when such are used and include chromic acid, the alkali metalchromates and dichromates such as sodium, potassium, lithium chromatesand dichromates and the like, the alkaline earth metal chromates anddichromates, such as magnes ium, calcium, strontium and barium chromatesand dichromates and the like, including mixtures of the foregoing. Whenusing the chromates and dichromates, it is preferred that such be usedwith a strong inorganic acid,

particularly sulfuric acid, and preferably on about a mole per molebasis.

The oxidizing agents of the present invention can be need in thetreatment of, in general, any sludge. As previously mentioned, thesludge can be treated either during the formation thereof or after thesludge has been formed. In general the sludge is formed when thephosphorus component of the phosphorus-containing gases is condensed tothe liquid form. The phosphoruscontaining gases can vary in composition,however, usually such gases contain, in addition to phosphorus gas, amajor amount, i.e., over about 50% by volume, of carbon monoxide and aminor amount of such materials as particulate impurities, methane,hydrogen, silicon tetrafluoride and the like. It is not uncommon for thephosphorus-containing gases to contain from about 1% to 15% by volume ofphosphorus and as much as 75% to 90% by volume of carbon monoxide.Treating the sludge during its formation can be accomplished by condensing the phosphorus with an aqueous medium containing the oxidizingagent, the temperature of which is below the dew point of thephosphorus. The pH of the aqueous medium used in condensing thephosphorus can vary but usually has a pH below about 7.5 and preferablyfrom about 1 to 5.5 and can contain, in addition to the oxidizing agent,ammonia, alkali metal and ammonium hydroxides and carbonates, and otherinorganic salts, particularly buffering salts. The temperature of theaqueous medium containing the oxidizing agent should be below thecondensation temperature of the phosphorus vapor, and, although suchvaries with the composition of the phosphorus-containing gases, ispreferably from about 40 C. to about 75 C. The concentration of theoxidizing agent in the aqueous medium can vary considerably and, ingeneral, any concentration can be used which reduces the phosphoruscontent of the sludge without oxidizing substantial amounts ofphosphorus to higher oxidation states. Especially useful areconcentrations in the aqueous medium having an oxidizing abilityequivalent to that obtained from the presence therein of from about0.10% to 5.0% by weight of chromic acid. Such concentrations are, ingeneral, prefer ably dilute, i.e., below about 20% by weight, andespeciallyly preferred are concentrations from about 0.01% to 10% byweight. Although, in general, the phosphoruscontaining gases need onlybe contacted with the aqueous condensing medium containing the oxidizingagent it is preferred that such be carried out under conditions whichestablish intimate contact of the gases with the aqueous medium such asby spraying the gases with a spray of the aqueous medium and the like.It should also be noted, that in some instances it may be advantageousto separately add the oxidizing agent during or after the contacting ofthe phosphorus-containing gases with the aqueous condensing medium.After treatment, the re covery of the phosphorus can be carried out bycollecting the liquid phosphorus containing materials, which includephosphorus, sludge and the aqueous condensing medium containing theoxidizing agent, allowing the phosphorus to settle out and collectingthe phosphorus.

Another method for treating the sludge is by contacting the sludge withan oxidizing agent after it has been formed. As previously mentioned,the sludge is, in general, a poorly defined emulsion or dispersioncontaining solid impurities, water and phosphorus and the content of thesludge can vary considerably in the amount of each material which ipresent. Therefore, no general sludge analysis can be given which coversall sludge compositions, although, usually a typical sludge which is aphosphorus in water type emulsion contains from about 1% to about 25%solids by weight, about 15 to 85% phosphorus by weight and the balancebeing water which is present at least above 5% by weight. Additionally,a typical sludge which is a Water in phosphorus type emulsion containsfrom about 1% to about 40% solids by weight, about 35% to about 98%phosphorus by weight and the balance being water which is present atleast above 1% by weight. The pH of the sludge can vary but is usuallybelow about 7.5 and more commonly from about 5 to 7.5. The sludge needonly be contacted with the oxidizing agent, however, it is preferredthat such be carried out under conditions which establish intimatecontact of the discontinuous phase of the sludge and the oxidizing agentsuch as by stirring, vigorous agitation and the like. In cases where thesludge contains appreciable amounts of water, the oxidizing agent can bedirectly added thereto, however, it may at times be advantageous tofirst dissolve the oxidizing agent in an aqueous solution and contactthe sludge therewith. In any event, it is usually desirable to contactthe sludge with the oxidizing agent in the presence of an aqueousmedium. The concentration of the oxidizing agent in the aqueous mediumcan vary considerably and, in general, any concentration can be usedwhich reduces the phosphorus content of the sludge without oxidizingsubstantial amounts of phosphorus to higher oxidation states. Especiallyuseful are concentrations in the aqueous medium having an oxidizingability equivalent to that obtained from the presence therein of fromabout 0.10% to 10.0% by weight of chromic acid. Such concentrations are,in general, preferably dilute, i.e., below about 20% by weight of theaqueous medium, and especially preferred are concentrations from about0.01% to 10% by weight of the aqueous medium. After treatment therecovery of the phosphorus can be carried out by allowing the phosphorusto settle out of the sludge and collecting the phosphorus.

In order to facilitate the description and understanding of thisinvention, reference is made to the appended drawings in which:

FIGURE 1 is a schematic flow sheet showing one embodiment of the instantinvention for the recovery of phosphorus from sludge;

FIGURE 2 is a schematic flow sheet showing another embodiment of theinstant invention for the recovery of phosphorus from sludge; and

FIGURE 3 is a schematic flow sheet showing still another embodiment ofthe instant invention for the recovery of phosphorus from sludge.

Referring now to FIGURE 1 which illustrates one embodiment of thepresent invention. Phosphorus-containing gases produced, for example, inan electric furnace and, if desired, passed through process stages, suchas electrostatic precipitators which remove from the gases to variousdegrees some of the particulate impurities, are directed by line 1 tothe condenser 2 where the gases are contacted with a condensing medium,usually water and containing, if desired, such materials as dissolvedammonia, sodium hydroxide, sodium carbonate and the like. Thephosphorus-containing gases are sprayed at a temperature below the dewpoint of phosphorus with the condensing medium. This results in theproduction of a liquid dispersion which includes phosphorus, condensingmedium and sludge. The dispersion thus obtained is passed to a sump 3and allowed to stratify into three rather distinct phases i.e.,phosphorus, sludge and condensing medium, although the boundariesbetween such phases are not necessarily distinct. The uncondensed gasesare discharged by line 4 from the system and are usually conveyed tovarious points in the plant to be used as fuel. The condensing medium ispassed by line 5 either for recycling back to the condenser or fordisposal. The phosphorus is recovered from the sump by passing throughline 6 to, for example, storage tanks maintained at a temperature highenough to keep the element in liquid form. The sludge phase which ismost generally a phosphorus in water type emulsion or dispersion ispassed through line 7 to a sludge m'uier tank 8. In this tank the sludgeis mixed with the oxidizing agent in the presence of a sufficient amountof aqueous medium in order to effect a separation of the phosphorustherefrom. As shown the mixer tank is operated continuously with thephosphorus separated from the sludge being removed therefrom by agravity outlet 9 to, for example, a storage tank maintained at anelevated temperature sufiicient to keep the phosphorus in the liquidform. The treated sludge is removed from the mixer tank by overflow andpassed by line for disposal.

This embodiment of the present invention will now be further illustratedby the following examples in which all parts and percentages are byweight unless otherwise specified.

EXAMPLE 1 A sludge phase (phosphorus in water type emulsion) from thesump obtained as hereinbefore described, having a density of 1.25 andthe following composition:

Composition: Percent by weight P 34 Solids 13 Water 53 was continuouslycharged at a rate of about 45 gal/hr. to a mixing tank under stirringwith a blade speed of about 105 r.p.m. A 1% chromic acid solution wascontinuously charged to the mix tank at about 90 gal/hr. The treatedsludge was recovered as overflow from the tank and the phosphorus wascontinuously withdrawn from an outlet in the bottom of the tank. Samplesof each taken at intervals of about 6 hours gave the following results:

TABLE 1 Treated sludge analysis Pl analysis CrOa Sample usage, lbs.

Percent Percent Percent Percent Percent CrOg/lOO P olids H2O CrO lbs P 1Percent B.I. (benzene insoluble.

Materials obtained by extracting with benzene phosphorus from thebenzene insoluble residue contained therein and expressed as percent byweight of phosphorus.

EXAMPLE 2 Another sludge phase (phosphorus in water type emulsion) fromthe sump obtained as hereinbefore described having a density of 1.42 andthe following composition:

Composition: Percent by weight P 60 Solids 9 Water 31 TABLE 2 Treatedsludge analysis P4 analysis ClOa Sample usage, lbs. N 0. Percent PercentPercent Percent Percent 01'03/100 P, solids water 010 B1. lbs. P4

The above data dramatically illustrates the ability of the presentinvention to reduce the phosphorus content of the various typicalsludges and, as can be observed therefrom, a considerable reduction ofphosphorus retained in the sludges is effected by the use of minoramounts of the oxidizing agent, chromic acid, as well as the recovery ofa relatively high quality phosphorus.

Results somewhat comparable to the foregoing examples can also beobtained under substantially similar conditions using other oxidizingagents which include hydrogen peroxide, nitric acid and alkali metalchlorates such as sodium chlorate, and good results comparable to theforegoing examples can be obtained with other watersoluble hexapositivechromium compounds such as the alkali metal and alkaline earth metalchromates and dichromates especially when such chromates and dichromatesare used with sulfuric acid.

Referring now to FIGURE 2 which illustrates another embodiment of thepresent invention. Phosphorus-containing gases produced, for example, inan electric furnace and, if desired, passed through electricprecipitators which remove from the gases to various degrees some of theparticulated impurities, are directed by line 1 to the condenser 2 whenthe gases are contacted with a condensing medium, usually water andcontaining, if desired, such materials as dissolved ammonia, sodiumhydroxide, sodium carbonate and the like. The phosphorus-containinggases are sprayed at a temperature below the dew point of phosphoruswith the condensing medium. This results in the production of a liquiddispersion which includes phosphorus, condensing medium and sludge. Thedispersion thus obtained is passed to a sump 3 and allowed to stratifyinto three rather distinct phases, i.e., phosphorus, sludge andcondensing medium, although the boundaries between such phases are notnecessarily distinct. The uncondensed gases are discharged by line 4from the system and are usually conveyed to various points in the plantto be used as fuel. The condensing medium is passed by line 5 either forrecycling back to the condenser or for disposal. The phosphorus isrecovered from the sump by passing through line 6 to, for example,storage tanks maintained at a temperature high enough to keep theelement in the liquid form. The sludges phase which is most generally aphosphorus in water type emulsion or dispersion is passed through line 7to a centrifuge 8. Some of the phosphorus is removed from the sludge inthe centrifuge and phosphorus-reduced sludge is passed therefrom throughline 9 for disposal. In some cases it may be necessary to add water tothe centrifuge in order to obtain better separation of the phosphorusfrom the sludge. The phosphorus from the centrifuge contains sludge,that is, water and particulate impurities, and is most generally a waterin phosphorus type emulsion. This material is passed through line 10 toa mixer tank 11. In this tank the phosphorus (containing sludgeimpuirties) is mixed with the oxidizing agent in the presence of asufficient amount of aqueous medium in order to effect a separation ofthe phosphorus from the sludge impurities contained therein. As shownthe mixer tank is operated continuously With the phosphorus separatedfrom the sludge impurities being removed therefrom by a gravity outlet12 to a storage tank maintained at an elevated temperature sufiicient tokeep the phosphorus in liquid form. The sludge impurities are removedfrom the mixer tank by overflow and passed by line 13 for disposal.

An alternate method to the foregoing, which in some cases may proveadvantageous, is to pass the phosphorus containing sludge impurities toa settling tank, allowing a portion of the phosphorus to separate fromthe sludge impurities by settling out, removing the remaining phosphoruscontaining sludge impurities which is most generally a water inphosphorus type emulsion and treating this emulsion with an oxidizingagent in a mixer tank as hereinbefore described.

Referring now to FIGURE 3 which illustrates another embodiment of thepresent invention. Phosphorus-containing gases produced, for example, inan electric furnace and, if desired passed through electricprecipitators which remove from the gases to various degrees some of theparticulated impurities, are directed by line 1 to the condenser 2 wherethe gases are contacted with a condensing medium made up of water and anoxidizing agent and containing, if desired, other additives, such as,dissolved ammonia, sodium hydroxide, sodium carbonate, and the like. Thephosphorus-containing gases are sprayed at a temperature below the dewpoint of phosphorus with the condensing medium. This results in theproduction of a liquid dispersion which includes phosphorus, condensingmedium and sludge. The dispersion thus obtained is passed to a sump 3and allowed to stratify into three rather distinct phases, i.e.,phosphorus, sludge and condensing medium, although the boundariesbetween such phases are not necessarily distinct. The uncondensed gasesare discharged by line 4 from the system and are usually conveyed tovarious points in the plant to be used as fuel. The condensing medium ispassed by line 5 either for recycling back to the condenser or fordisposal. The phosphorus is removed from the sump by passing throughline 6 to, for example, storage tanks maintained at a temperature highenough to keep the element in the liquid form. The treated sludge isremoved from the sump through line 7 for disposal. In the foregoingembodiment, the use of an oxidizing agent in the condenser mediumresults in a reduction of phosphorus retained in the sludge.

It can, of course, be appreciated that various combinations of theforegoing as well as other embodiments may prove advantageous in certaininstances, as, for example, the use of an aqueous solution of theoxidizing agent as the condensing medium in condensing the phosphorus inthe phosphorus-containing gases to the liquid form as well as treatingthe sludge obtained therefrom with a solution of an oxidizing agent toreduce the phosphorus content of the sludge.

The process of the present invention has many advantages over theconventional physical methods for recovering phosphorus from the sludgewhich includes an easy, economical and simple method which does notrequire elaborate processing equipment along with the necessary care andmaintenance thereof as Well as permitting in some instances processequipment to be eliminated such as, for example, the elimination of theelectrostatic precipitators used in purifying the phosphorus-containinggases. Additionally, the process is quite versatile in that it can beadapted for use at various stages in the process. Still further, theprocess permits the recovery of a relatively high quality phosphorus.

What is claimed is:

1. A process for reducing the phosphorus content of a sludge whichcomprises incorporating a Water-soluble inorganic hexavalentoxy-chromium compound into said sludge and intimately contacting thediscontinuous phase of said sludge with said compound to improve theseparation of the phosphorus contained in said sludge.

2. A process according to claim 1 wherein said compound is chromic acid.

3. A process for reducing the phosphorus contents of sludge comprisingadmixing said sludge and a water-soluble inorganic hexavalentoxy-chromium compound in the presence of an aqueous medium, saidcompound being present at a concentration of below about 20% by weight,and separating phosophorus from said sludge.

4. A process according to claim 3 wherein said compound is chromic acid.

5. A process according to claim 4, wherein said chromic acid is presentin concentrations of from about 0.01% to 10.0% by weight.

6. In the process for preparing elemental phosphorus wherein phosphaticmaterials are thermally reduced with a reducing agent in the presence ofa flux to produce phosphorus-containing gases and thereafter condensingsaid phosphorus in said gases by spraying said gases with an aqueouscondensing medium having a temperature below the dew point ofphosphorus, collecting the liquid phosphorus-containing materialstherefrom, separating said materials into a phosphorus-containing sludgephase and a phosphorus phase and separating said phases, the improvementwhich comprises reducing the phosphorus content of sludge :by admixing,in the presence of an aqueous medium, said sludge with chromic acid, andrecovering phosphorus therefrom.

7. In the process for preparing elemental phosphorus wherein phosphaticmaterials are thermally reduced with a reducing agent in the presence ofa flux to produce phosphorus-containing gases by spraying said gaseswith an aqueous condensing medium having a temperature below the dewpoint of phosphorus, collecting the liquid phosphorus-containingmaterials therefrom, separating said materials into a sludge phase and aphosphorus phase, separating said phases and centrifuging said sludgephase to recover phosphorus-containing sludge impurities therefrom, theimprovement which comprises reducing the phosphorus content of thephosphorus-containing sludge impurities by admixing, in the presence ofan aqueous medium, said phosphorus-containing sludge impurities withchromic acid, and recovering phosphorus therefrom.

References Cited UNITED STATES PATENTS 1,856,144 5/1932 Wietzel 231653,084,029 4/ 1963 Barber et al 23-223 3,113,839 12/1963 Barber et al23-223 XR 3,152,870 10/1964 Baumgartner et al. 23-219 3,318,665 5/1967Schmidt et al. 23-219 3,104,952 9/1963 Hartig 23293 FOREIGN PATENTS524,289 8/1940 Great Britain. 624,307 4/ 1963 Belgium.

EDWARD STERN, Primary Examiner.

US. Cl. X.R. 23-293, 312

